One example of a conventional sliding seal is a ring packing (see PTL 1, for example). This ring packing is used to appropriately manage sliding resistance between a tubular body and a shaft body. The tubular body slidably supports the shaft body which is displaced in conjunction with a diaphragm. The ring packing is attached to an outer periphery of the shaft body. The ring packing has an oval cross section. A concave groove is formed on an outer peripheral surface of the ring packing in a circumferential direction, the outer peripheral surface being opposed to an inner surface of the tubular body.
Grease is applied to an entire periphery of the ring packing, and especially, the grease is adequately applied to an inside of the concave groove. With this, even when the ring packing repeatedly slides in a vibrating manner by the vibrations of the diaphragm, the grease remains in the concave groove. Therefore, a sharp increase in the sliding resistance can be prevented.